Yes, definitely. However, there may be some performance issues separate from whether or not it is a mirrored volume set. If you add 9 GB drives to a volume set with 2GB or 4GB disks (with no reload) and the environment is reasonably static then the data that you access is still spread across all disk drives reasonably well. However, if you reload the volume set you must keep in mind that MPE/iX writes data to the disk drive on the volume set (excluding ldev 1, which is a special case) with the most available free space. This means the first data restored will not be spread but will be written to the 9GB disk(s). After the free space on the 9GB disk(s) has been adequately utilized, the remaining data is spread across all of the other drives. This can create a potential I/O bottleneck with the 9 GB disk and impact performance. It all depends upon the I/O demand on your system.